An EL device is that holes and electrons, injected from two facing electrodes, combine with each other in a luminescent layer so as to generate energy by which a luminescent material in the luminescent layer is excited to emit light whose color depends on the luminescent material. EL devices are widely used for displays, lighting systems, etc. as panel light-emitting devices of self-emission type.
In recent years, light-emitting devices comprising luminescent layers containing semiconductor quantum dots have been proposed and developed. A quantum dot is a crystal composed of two or more semiconductor atoms, having a size in the order of several nanometers to several tens nanometers. In such a nanometer-sized small crystal, electrons do not form a continuous band structure but exist in discrete energy levels. That is to say, since such small crystals remarkably show the quantum size effect, their electron confinement effect is greater than that of bulk crystals, which are larger than quantum dots in size. Thus, nanometer-sized crystals can increase the probability of recombination of excitons.
Further, in light-emitting devices using quantum dots, it is possible to regulate emission frequency without changing their constitution. Because of its quantum confinement effect, a quantum dot has optical properties that are dependent on its size. For example, it is possible to change the luminescent color of a quantum dot consisting of CdSe from blue to red by merely changing the size of the quantum dot. Furthermore, a quantum dot has a relatively narrow spectrum half width, and the half width can be made less than 30 nm, for example. It can therefore be said that quantum dots are excellent as materials for luminescent layers.
Quantum dots are also called nanocrystals, fine particles, colloids, clusters, etc. In this Specification, those crystals, particles, and the like that have the quantum size effect are all included in quantum dots.
Known as processes for forming luminescent layers using such quantum dots include spin coating and dip coating that use a colloidal solution containing quantum dots having a ligand, such as tri-n-octylphosphine oxide (TOPO), attached to their surfaces (see JP 2005-522005T and JP 2006-520077T, for example). the ligand attaches to the surfaces of the quantum dots, and makes the dispersion stability of the quantum dots high.
In a conventional luminescent layer using quantum dots having a ligand, such as TOPO, attached to their surfaces, however, the quantum dots are not stable, which may affect the life characteristics of the luminescent layer. Especially when the quantum dots are phosphorescent, the life characteristics of the luminescent layer are apt to be affected because phosphorescent materials are longer in life than fluorescent ones.